1. Field of the Invention
This invention relates generally to a raster scanning device, such as an inkjet printer of the sort that constructs images as arrays of very large numbers of individually computer-controlled inkdrops on a printing medium that is computer-advanced in very small steps through the printer; and more particularly to encoder apparatus for very accurately advancing, or controlling the advance of, the printing medium through the printer.
Some large-format printers of this sort are sometimes called "plotters". For purposes of this document, except as indicated by context, the terms "printers", "printing devices" etc. encompass such plotters.
2. Related Art
In this field it is known to use position-encoding devices, or in abbreviated form "encoders", to help establish the position of a piece of printing medium relative to inkdrop-expelling modules, often called "pens" or "printheads", of a printer. An encoder generally has two main elements that are subject to relative movement.
One of these elements is--in one manner or another--extended along the direction of relative movement and has graduations that are, in effect, arrayed along that direction of movement. The other element is positioned to sense relative passage of such a graduation and in response produce some sort of signal that is expressed as or developed into a digital electronic signal.
In one type of encoder for a rotary-drive system, visible graduations are arrayed about the shaft or hub of a rotary drive element (a roller or platen), that directly engages and advances the printing medium; and an optical sensor is disclosed to respond with an electrical pulse to passage of each graduation. In such a rotary-drive system the only element that may be said to move linearly is the print medium itself.
If preferred instead, a linear drive element may be included--a print-medium-carrying bed that engages, holds and advances the medium. In this linear-drive case, graduations may be arrayed along the longitudinal extent of the bed; a sensor is disposed to respond to each graduation, generally as in the rotary case.
It will be understood that in most such systems whether linear or purely rotary, in purest principle it is immaterial whether the graduations are fixed relative to the moving drive element (platen or bed) and the sensor is stationary with respect to the rest of the printer, or the sensor is fixed to and rides on the moving drive element while the graduations are stationary. Hence all such topological inversions shall for purposes of this document be considered equivalents.
As suggested above, graduations may be primarily only visible features--such as painted or etched marks--or may partake of a more mechanical character as in the case of grooves, apertures or raised ribs. Naturally, the type of sensor employed varies accordingly.
One special case of well-known rotary encoder uses only one single graduation, which gives rise to just one sensor pulse for each rotation of the associated shaft. The graduation used in such a system may be a magnet fixed to a rotating shaft, and the sensor may be another magnetic element such as a second magnet or a coil of wire, mounted to respond mechanically or electrically to the rotating magnet.
Now with graduations arrayed along a linear drive element such as a print-medium-carrying bed, the relative movement of the medium with respect to the rest of the printer has a very simple relationship to the relative movement of the encoder sensor with respect to the encoder graduations. That relationship is one-to-one.
As a result, in such a linear encoder with one of the two elements (graduation array and sensor) essentially fixed relative to the printing medium and the other fixed to the rest of the printer, the precision of position determinations along the advance direction of the medium is limited by the resolution of the encoder system.
That resolution is the ability of the system to properly and reliably distinguish each graduation from the adjacent ones. This ability may also be described as the readability of the graduations through interpretation of the sensor pulses, or the precision with which the sensor pulses correspond to the passage of graduations past the sensor position.
In addition to these precision considerations, the accuracy of position determinations along the advance direction of the medium is limited by the positional accuracy of the encoder-system graduations. In the relatively simple case of a linear drive system, overall precision and accuracy of such positional determinations are not only set by but essentially equal to the precision and accuracy of the encoder system.
This relationship is relatively undesirable, for the desired printing precision is on the order of a small fraction of one millimeter (roughly 0.008 mm, or about 0.0003 inch). Such fine-resolution linear encoders are feasible, and in fact are used for printhead positioning in the direction transverse to printing-medium advance, in printers of the type under consideration.
Even for motion along that transverse direction (which is usually much shorter than the print-medium advance), such encoder systems are relatively very expensive, generally requiring very finely spaced graduations--as for example in the form of very narrow apertures etched in a metallic strip--and two sensors very precisely spaced apart and read in quadrature to effectively interpolate between those fine graduations. For the print-medium advance direction an even longer array of graduations would be required.
In any event such a system could be practical for a driven-linear-bed printer, but currently such mechanisms are disfavored for economic reasons in ordinary commercial printers, and in printers that make very long engineering-size drawings as for instance on continuous paper rolls. Current practice favors mechanisms that drive the printing medium itself--with no relatively costly and heavy suspended movable platform or bed--through the printer by rollers or around platens.
It is known in the art to use an encoder to establish position in such a rotary-drive system, with one of the encoder elements fixed directly to the shaft or hub of a drive platen and the graduations arrayed about the platen axis. (For some purposes of this document it will be convenient to refer to such an encoder in a verbal shorthand as a "direct-coupled encoder" or in even more-abbreviated form a "direct encoder".) If this scale is radially positioned near the platen or roller circumferential surface, the resolution along the print medium, as in the linear case is essentially equal to the linear resolution of the encoder system itself--which is to say, ordinarily, the spacing of the encoder graduations.
In a rotary system the angular resolution of the encoder system and thereby the linear resolution along the print medium can be improved by placing the array of graduations--and its associated sensor--at a greater radius from the platen or roller axis. With larger radius one can provide a greater number of divisions, or readable divisions, in each rotation of the shaft. The resulting improvement in fineness of linear resolution along the medium is proportional to the ratio, or multiple, of graduation-and-sensor radius relative to platen radius.
As a practical matter, however, this multiple is limited by available space within the printer case; moreover, problems of concentricity can become significant with increasing radius. Also a large graduated disc may introduce new concerns such as cost of manufacture, or mechanical and thermal stability.
Further, in a rotary system new variables come into play; one of these is the systematic error introduced by effective radius of the printing-medium surface on which the printer makes marks. The effective radius is influenced by thickness of the medium itself, and by manufacturing tolerances and, in principle, wear in the platen.
Another variable is circumferential slippage of the medium relative to the platen. It is known to provide means for measurement of the aggregate effect of these variables in situ by a printer user in the field, and to program a microcomputer which controls each printer to compensate for these measured variables by taking them into account in calculating position along the medium-advance direction.
Thus in one printer that is commercially available from the Hewlett Packard Company, marks are made automatically by the printer along the pen-advance direction--at right angles to medium advance. These marks are made on a piece of the same printing medium that is to be used in accurately-positioned printing along the medium-advance direction, to form a special, customized scale.
The printer user then rotates the scale-printed piece of printing medium through a right angle and reinserts the piece, thus oriented, for passage through the printer in the ordinary advance mode. The printer has optical sensors for finding the custom-scale marks, and its control computer has programming for using the marks to determine the composite effects of diametral tolerance (and theoretically wear), and slippage, to develop a calibration table for use in later operation to correct the information provided by the encoder system.
Such a system has the important advantage of compensating for print-medium thickness and wear--systematic factors which affect accuracy and which cannot be known when the printer is manufactured. It also corrects for limited other kinds of systematic errors, such as cyclical errors due to warping of a platen or drive roller and due to eccentric placement of the encoder scale relative to the platen or roller axis.
As will be clear, nevertheless, this system is somewhat awkward to use and in any event cannot improve the resolution or precision of a medium-advance-direction encoder system, beyond the resolution and precision which are mechanically inherent in it.
Countering the larger-radius/greater-number-of-divisions approach is a philosophically opposite one, embodied in the single-graduation type of rotary system mentioned earlier. Since just one pulse is produced for each shaft rotation, and a typical printing-medium platen or drive roller has radius between about 5 mm to 3 cm, poor resolution (about 1% to 9 cm) would result from placing such a system directly on the platen or drive-roller shaft.
Even with modification to count two or four pulses per shaft rotation (as for example by counting both ends of a permanent magnet mounted crosswise to the shaft), such resolution is entirely inadequate for modern purposes in which desired resolution amounts to small fractions of a millimeter.
Therefore it is common to place such single-graduation (or small-number-of-graduation) encoders on shafts that operate at a very large mechanical advantage relative to the shaft whose position is to be measured. For example such encoders may be placed on shafts that are linked by belt or gear drives that provide a mechanical advantage of 100:1 to perhaps 10,000:1. (For verbal-shorthand purposes in this document such an encoder will sometimes be called a "remote-coupled encoder" or even more simply a "remote encoder".)
To avoid the incremental cost of providing such a drive for positional measurement exclusively, it is known to mount such an encoder to the shaft of a motor that is in the system anyway--i.e., a motor that drives the printing-medium platen or roller--and that is linked to the platen through a gearbox that is likewise in the system already.
In this case a further subdivision of each rotation by a medium-size factor (for example, perhaps eight to sixty-four) can be obtained through use of a stepping motor. A stepping motor is in effect a special case of a magnetic encoder, since the armature and stationary coils of such a motor provide--in addition to motive force--a rotation-counting (or partial-rotation-counting) function that is equivalent to the response of an encoder coil to its rotating magnet. To that extent this type of motor is in essence self-encoding, but at significant added cost.
Whether a stepping motor or a separate graduation-and-sensor encoder is used, the interposition of a gearbox or other means for providing a mechanical advantage introduces still other undesirable effects. These are various phenomena that can lead to imprecise and inaccurate correspondence between advance of the encoder (or self-encoder) count and the theoretically corresponding linear advance of the printing medium.
Foremost among these are cyclical errors arising from eccentric or otherwise imperfect gears. Some more insidious effects can intrude, such as--in bidirectional medium-advance systems for instance--inconsistent takeup of backlash. Thus while medium-advance-direction encoders have been used in sophisticated ways heretofore, such use has not focused on dealing with resolution and precision, or with gear-train-generated cyclical errors and their related problems.
From all this it can be summarized that systems in which encoder elements move nearly directly with the printing medium--while highly accurate--are subject to relatively poor resolution; whereas systems in which encoder elements move with a high mechanical advantage relative to the printing medium, while offering fine resolution, are subject to unacceptable systematic inaccuracies. The first type of system can be rendered technically acceptable only through use of relatively expensive, high-resolution encoders; while the second type can be rendered technically acceptable only through use of relatively very expensive high-precision gearing.
Even in very expensive encoders, cyclical errors of still another type are typically present: errors in or associated with the encoder discs. These errors are due to mounting eccentricity, mounting perpendicularity, cyclical errors in mastering equipment used for generating an original pattern of graduations (which may later be replicated myriad times as by silkscreening or photoetching), and other contributors. In earlier systems these error sources can be controlled only by high tolerancing and careful, expensive mounting technique.
As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement, as no system has been introduced that offers both high resolution and good systematic accuracy at relatively modest cost.